The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950.degree. C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Heroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by anthracite-based or graphite carbon blocks at the wall and at the cell floor bottom which acts as cathode and to which the negative pole of a direct current source is connected by means of steel conductor bars embedded in the carbon blocks.
The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still approximately 950.degree. C. in order to have a sufficiently high rate of dissolution of alumina which decreases at lower temperatures and to have a higher conductivity of the electrolyte.
The carbonaceous materials used in Hall-Heroult cells as cell lining deteriorate under the existing adverse operating conditions and limit the cell life.
The anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form CO.sub.2 and small amounts of CO. The actual consumption of the anode is approximately 450 kg/ton of aluminium produced which is more than 1/3 higher than the theoretical amount.
The carbon lining of the cathode bottom has a useful life of a few years after which the operation of the entire cell must be stopped and the cell relined at great cost. Despite an aluminium pool having a thickness of 15 to 20 cm maintained over the cathode, the deterioration of the cathode carbon blocks cannot be avoided because of penetration of sodium into the carbon which by chemical reaction and intercalation causes swelling, deformation and disintegration of the cathode carbon blocks, as well as penetration of cryolite and liquid aluminium.
The carbon blocks of the cell side wall do not resist oxidation and attack by cryolite and a layer of solidified cryolite has to be maintained on the cell side walls to protect them. In addition, when cells are rebuilt, there are problems of disposal of the carbon cathodes which contain toxic compounds including cyanides.
Another major drawback, however, is due to the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD), also called interelectrode gap (IEG), must be kept at a safe minimum value of approximately 50 mm to avoid short circuiting between the aluminium cathode and the anode or reoxidation of the metal by contact with the CO.sub.2 gas formed at the anode surface.
The high electrical resistivity of the electrolyte, which is about 0.4 ohm. cm., causes a voltage drop which alone represents more than 40% of the total voltage drop with a resulting energy efficiency which reaches only 25% in the most modern cells. The high cost of energy together with the low efficiency, has become an even bigger item in the total manufacturing cost of aluminium since the oil crisis, and has decreased the rate of growth of this important metal.
In the second largest electrochemical industry following aluminium, namely the caustic and chlorine industry, the invention of the dimensionally stable anodes (DSA.RTM.) based on noble metal activated titanium metal, which were developed around 1970, permitted a revolutionary progress in the chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine-caustic purity. The substitution of graphite anodes with DSA.RTM. increased drastically the life of the anodes and reduced substantially the cost of operating the cells. Rapid growth of the chlorine caustic industry was retarded only by ecological concerns.
In the case of aluminium production, pollution is not due to the aluminium produced, but to the materials and the manufacturing processes used and to the cell design and operation.
However, progress has been reported in the operation of modern aluminium plants which utilize cells where the gases emanating from the cells are in large part collected and adequately scrubbed and where the emission of highly polluting gases during the manufacture of the carbon anodes and cathodes is carefully controlled.
While progress has been reported in the fabrication of carbon cathodes by the application of coatings or layers using new aluminium wettable materials which are also a barrier to sodium penetration during electrolysis, no progress has been achieved in design of cathodes for aluminium production cells with a view to reducing the interelectrode gap and the rate of wear of its surface.
U.S. Pat. No. 4,560,488 to Sane et al discloses a recent development in molten salt electrolysis cells concerning making materials wettable by molten aluminium. However, the carbon or graphite anodes and cathodes are of conventional design with no suggestion leading to the present invention.
U.S. Pat. No. 4,681,671 to Duruz illustrates another improvement in molten salt electrolysis wherein operation at lower than usual temperatures is carried out utilizing permanent anodes, e.g. metal, alloy, ceramic or a metal-ceramic composite as disclosed in European Patent Application No. 0030834 and U.S. Pat. No. 4,397,729. Again, while improved operation is achieved at lower temperatures, there is no suggestion of the subject matter of the present invention.
PCT Application WO 89/06289 to La Camera et al deals with an improved molten electrolysis wherein attention is directed to an electrode having increased surface area. However, again, there is no disclosure leading one to the present invention.
The following references disclose several other attempts to improve cell operation.
European Patent Application No. 0308015 to de Nora discloses a novel current collector;
European Patent Application No. 0308013 to de Nora deals with a novel composite cell bottom; and
European Patent Application No. 0132031 to Dewing provides a novel cell lining.
U.S. Pat. No. 5,203,971 discloses an aluminium electrowinning cell having a partly refractory and partly carbon based cell lining. The carbon-based part of the cell bottom may be recessed in respect to the refractory part.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none deal with or suggest the present invention.